The invention concerns a circuit arrangement for the supply of an electrical coil, particularly the coil of a solenoid valve, with a predetermined operating current from a voltage source with differing operating voltages, the coil being arranged in series with a first controllable circuit element and a current sensor, and the measuring signal of the current sensor being compared with a reference value through a first comparator, and a timed switch being triggered by the output signal of the comparator when the measuring signal exceeds said reference value, the first circuit element, activated by an operating signal, being disconnected by the output signal of the timed switch, until the measuring signal exceeds the reference value again.
A commercially available circuit arrangement of this kind has the embodiment shown in FIG. 3. It serves the purpose of enabling the operation of the magnet coil of a solenoid valve with different operating voltages, for example with 12 Volt or 24 Volt, at the same operating current, as it is not always possible to operate the same coil over a large operating voltage range of, for example, 9 to 32 Volt: The output supplied at high operating voltage could be too high (it increases with the square of the voltage). To enable the use of the same coil in a large voltage range, for example from 9 to 32 Volt, thus avoiding the use of different coils for the different operating voltages, the known circuit arrangement keeps the current flowing through the coil L constant, independently of the actual operating voltage Ub. For this purpose, a transistor T2 operated as circuit element and a current sensor in the form of measuring resistor R are arranged in series with the coil L. The current measuring signal in the shape of the voltage drop caused by the current I at the resistor R is compared in a comparator 1 with a reference value Uref being substantially equal to the desired value of the current. The output signal A occurring at the outlet of the comparator 1, when the current measuring signal exceeds the reference value, triggers a timed switch in the shape of a monostable multivibrator 2, whose output signal is linked with an operating signal M through an AND-link 3. The outlet of the AND-link 3 is connected with the control connection of the transistor T2. The operating signal M occurs for as long as the solenoid valve must operate, that is, the current I has to flow. Thus, before begin of operation, the current measuring signal is lower than the reference value Uref, so that a high signal appears at the reversing outlet of the multivibrator 2 and the transistor T2 is ON (connected). As soon as the current I, increasing according to an exponential function, exceeds the reference value, a high signal A appears at the outlet of the comparator 1, which signal triggers the monostable multivibrator 2 and disconnects (blocks) the transistor T2 via the AND-link 3.
The diagrams in FIG. 4 show the mode of operation of the circuit arrangement according to FIG. 3. For the duration of the operating signal M, the current I flows. During the time t1, it increases exponentially until it reaches a peak value Ip, at which the reference value Uref is exceeded. The resulting signal A of the comparator 1 immediately blocks the transistor T2 again. However, the current I in the coil L continues to flow through a freewheeling diode D1 and a transistor T1 connected anti-parallel to the coil L and a Z-diode DZ1, which transistor T1 is ON (connected) for the duration of the high operating signal M. However, in this connection, the current I also drops according to an exponential function during the disconnection duration taus of the transistor T2 immediately after the disappearance of the output signal A. The monostable multivibrator 2, triggered again immediately by the disappearance of the output signal A, connects the transistor T2 again immediately after the disconnection duration taus of the transistor T2 determined by the cycling time of the monostable multivibrator 2. Then, the procedure described repeats itself, until the operating signal M disappears. At the end of the operating signal M, both transistors T1 and T2 are OFF (disconnected), after which the current I flows via the freewheeling diode D1 and the Z-diode DZ1 and quickly disappears.
The actual value of the current I is then only slightly below the peak value Ip=Uref/R and corresponds to the average value of the approximately sawtooth shaped course of the current I in FIG. 4.
The increasing time of the current I, however, depends on the size of the operating voltage Ub. As the increasing time t1 determines the response speed of the solenoid valve, the function of the solenoid valve also depends on the size of the operating voltage.
The invention is therefore based on the task of providing a circuit arrangement as described in the introduction, in which the increasing time of the current flowing through the coil after begin of operation is independent of the operating voltage.
According to the invention, this task is solved in that through a second comparator the measuring signal of the current sensor is compared with a signal that is steadily increasing independently of a change of the operating voltage, said signal also being triggered through the operating signal, and the timed switch is triggered by the output signal of the second comparator, when the measuring signal exceeds the steadily increasing signal, before the measuring signal exceeds the reference value.
With this solution, the increasing time of the coil current, immediately after begin of operation until reaching the reference value, is determined by a constant increasing time of the steadily increasing signal and is therefore constant.
In a simple way, the steadily increasing signal can be the output signal of an integrator with constant input signal.
Thus, the integrator may have a capacitor on the outlet of a constant current generator.
Parallel to the capacitor, a second circuit element may be arranged that can be disconnected on the appearance of an operating signal, and vice versa.
Further, it may be ensured that the outlets of the comparators are connected with the inlet of the timed switch via an OR-link and the output signal of the timed switch is linked with the operating signal through an AND-link, whose outlet is connected with a control connection of the first circuit element.